Spin ice in a general applied magnetic field: Kasteleyn transition, magnetic torque and rotational magnetocaloric effect

TL;DR

This paper explores the behavior of spin ice under arbitrary magnetic fields, revealing the presence of the Kasteleyn transition, magnetic torque effects, and a rotational magnetocaloric effect, extending understanding beyond high-symmetry field directions.

Contribution

It generalizes the Kasteleyn transition to arbitrary field directions in spin ice and analyzes associated thermodynamic and magnetic properties.

Findings

Kasteleyn transition occurs for general field directions.

The Kasteleyn temperature $T_K$ depends logarithmically on field angle.

A significant rotational magnetocaloric effect is observed.

Abstract

Spin ice is a paradigmatic frustrated system famous for the emergence of magnetic monopoles and a large magnetic entropy at low temperatures. It exhibits unusual behavior in the presence of an external magnetic field as a result of the competition between the spin ice entropy and the Zeeman energy. Studies of this have generally focused on fields applied along high symmetry directions: [111], [001], and [110]. Here we consider a model of spin ice with external field in an arbitrary direction. We find that the Kasteleyn transition known for $[001]$ fields, appears also for general field directions and calculate the associated Kasteleyn temperature $T_K$ as a function of field direction. $T_K$ is found to vanish, with a logarithmic dependence on field angle, approaching certain lines of special field directions. We further investigate the thermodynamic properties of spin ice for $T>T_K$, using a Husimi cactus approximation. As the system is cooled towards $T_K$ a large magnetic torque appears, tending to align the $[001]$ crystal axis with the external field. The model also exhibits a rotational magnetocaloric effect: significant temperature changes can be obtained by adabiatically rotating the crystal relative to a fixed field.